Introduction and Context
Colorectal polyposis is a condition characterized by the development of numerous precancerous adenomatous polyps in the colon and rectum. For decades, clinicians have relied on genetic testing for germline pathogenic variants in the *APC* gene to diagnose Familial Adenomatous Polyposis (FAP). However, a significant clinical gap remains: a substantial proportion of patients who present with a high number of adenomas do not harbor these germline mutations. These individuals, often classified as having ‘unexplained polyposis,’ face uncertainty regarding their cancer risk and the necessity of intensive surveillance for themselves and their relatives.
Recent advancements in genetic sequencing technologies have brought to light the phenomenon of ‘mosaicism.’ Unlike germline mutations, which are present in every cell of the body from conception, mosaic mutations occur after fertilization. This results in the pathogenic variant being present in only a subset of cells or tissues. Because standard genetic tests are often designed to detect mutations present in 50% or 100% of cells (germline), low-level mosaicism is frequently missed.
The study by Terlouw et al., published in *Gastroenterology*, addresses this gap by quantifying the prevalence of *APC* mosaicism and providing the first concrete guidelines for testing and managing these patients. This consensus marks a pivotal shift in how we approach hereditary colorectal cancer syndromes, moving from a binary ‘positive/negative’ germline view to a more nuanced understanding of genetic architecture.
New Guideline Highlights
The research provides a comprehensive look at 541 patients with a broad spectrum of polyposis phenotypes. The core finding is that *APC* mosaicism is far more common than previously suspected, particularly in those with ‘moderate’ polyposis.
**Key Takeaways for Clinicians:**
- **High Detection Rate:** Approximately 9.4% of patients with unexplained polyposis were found to have *APC* mosaicism.
- **Phenotype Matters:** The detection rate climbed to 14.3% in patients who met national guidelines for hereditary polyposis (typically those with 10–20+ adenomas).
- **The 20/30 Rule:** A new diagnostic threshold has been established: patients with ≥20 adenomas before age 60 or ≥30 adenomas before age 70 should be prioritized for mosaicism testing.
- **Extracolonic Manifestations:** Mosaic patients are at risk for more than just colon cancer; 26% of those studied also presented with gastroduodenal polyps.
Updated Recommendations and Key Changes
Previously, patients who tested negative for germline *APC* or *MUTYH* mutations were often left in a clinical ‘grey zone.’ The new expert consensus shifts the diagnostic paradigm toward targeted next-generation sequencing (NGS) with high depth of coverage to identify low-frequency variants.
| Feature | Previous Approach | New Consensus (2024/2026) |
| :— | :— | :— |
| **Genetic Testing** | Primarily focused on germline (50% VAF) variants. | Targeted NGS with high depth to detect mosaicism (low VAF). |
| **Testing Threshold** | Often restricted to patients with 100+ polyps or strong family history. | Testing recommended for ≥20 adenomas (<60y) or ≥30 adenomas (<70y). |
| **Surveillance** | Based on clinical symptoms; less standardized for mutation-negative patients. | Mandatory regular colonoscopy and at least one EGD for all mosaic patients. |
| **Offspring Risk** | Often dismissed if the parent's germline test was negative. | Consider testing offspring; recognize potential for germline transmission if mosaicism is present in germ cells. |
Topic-by-Topic Recommendations
1. Diagnostic Criteria and Risk Stratification
Experts recommend that clinicians no longer assume a negative germline test rules out *APC*-related disease. The ‘phenotype-first’ approach is essential. If a patient presents with a significant polyp burden that mimics FAP or Attenuated FAP (AFAP), they should be screened for mosaicism. The study specifically highlights that even patients with a relatively low number of adenomas (e.g., 10-19) may harbor mosaic variants, though the yield is higher in more severe phenotypes.
2. Surveillance Strategies
Once *APC* mosaicism is confirmed, the patient must be managed as having a hereditary cancer syndrome.
- **Colonoscopy:** Regular intervals are required. The frequency should be dictated by the polyp burden and the rate of polyp recurrence, similar to the management of AFAP.
- **Upper GI Surveillance:** Because 26% of mosaic patients develop gastroduodenal polyps, at least one esophagogastroduodenoscopy (EGD) is recommended. Follow-up EGDs should be based on the Spigelman stage of any polyps found.
3. Special Populations: Offspring and Reproductive Counseling
One of the most complex aspects of mosaicism is the risk to the next generation. If the mosaic mutation is present in the patient’s germ cells (sperm or oocytes), the child could inherit the mutation as a full germline variant, potentially resulting in classic FAP. The study found the mosaic variant in the semen of 50% of the small group tested. While none of the children in this specific cohort inherited the variant, the theoretical risk remains. Clinicians should discuss the possibility of germline testing for offspring, especially if the parental variant can be detected in different tissue types (e.g., blood and skin or saliva).
Expert Commentary and Insights
The consensus panel emphasizes that the diagnosis of mosaicism is a ‘technical challenge.’ Standard clinical NGS panels may filter out variants with a Variant Allele Frequency (VAF) of less than 10-15%, dismissing them as ‘noise’ or sequencing errors.
“The clinical implication is profound,” notes one researcher. “We are finally giving a name and a management plan to patients who were previously told their polyposis was ‘unexplained.’ By identifying *APC* mosaicism, we can provide precise surveillance that prevents cancer while also offering clearer answers to their families.”
However, experts also acknowledge a point of controversy: the cost-effectiveness of widespread mosaicism testing in patients with fewer than 10 adenomas. The detection rate in the ‘low-phenotype’ group (those not meeting current guidelines) was only 2.3%. Consequently, the current recommendation focuses on those with a higher adenoma count where the diagnostic yield justifies the resource expenditure.
Practical Implications
For the practicing gastroenterologist, these recommendations mean that a ‘negative genetic test’ from five or ten years ago may no longer be definitive. Patients with persistent or high-burden adenomas should be re-evaluated using modern, high-depth NGS techniques capable of detecting mosaicism.
**A Fictional Case Study: John’s Diagnosis**
John, a 58-year-old American male, had undergone multiple colonoscopies over the last decade, with a total of 25 adenomas removed. His initial genetic testing for *APC* and *MUTYH* five years ago was negative. Under the old paradigm, John was considered to have ‘sporadic’ polyps, albeit many of them. Following the new consensus, John’s physician ordered a high-depth NGS panel. The results showed an *APC* mosaic variant with a 4% VAF in his blood. This diagnosis changed John’s management: he was scheduled for his first EGD (which found two small duodenal polyps) and his children were counseled regarding their own potential risks. John now has a clear clinical pathway, moving from ‘unexplained’ to ‘managed.’
References
1. Terlouw D, et al. Prevalence and Consequences of APC Mosaicism in Patients With Colorectal Adenomas. *Gastroenterology*. 2026. PMID: 41801175.
2. Jasperson KW, et al. APC-Associated Polyposis Conditions. *GeneReviews®*. University of Washington, Seattle.
3. Syngal S, et al. ACG Clinical Guideline: Genetic Testing and Management of Hereditary Gastrointestinal Cancer Syndromes. *The American Journal of Gastroenterology*.
4. Monahan KJ, et al. UK guidelines for the management of hereditary colorectal cancer syndromes. *Gut*.
